Asymmetric optical interference measurement method and apparatus

ABSTRACT

An asymmetric optical interferometry method, comprises the following steps: an incident light is split into two beams, and the two beams are respectively projected onto a surface of an object to be tested and a reference mirror; and then respectively pass through a first imaging lens having a larger area on the side of the object, and a second imaging lens having a smaller area on the side of the reference mirror, and overlap on a photoelectric sensor after passing through a third imaging lens to form at least one interference image (S1); the corresponding interference image is input into a computer to obtain a signal of the corresponding interference image (S2); parse the signal of the corresponding interference image to obtain a three-dimensional shape (S3) of the surface of the object. Also provide an asymmetric optical interferometry device.

TECHNICAL FIELD

The present application relates to the technical field of precisionoptical measurement, and more particularly relates to an asymmetricoptical interferometry method and device.

BACKGROUND ART

In recent years, with the advancement and development of precisionmanufacturing technology, the technology for detecting the surfaceshapes of objects has been widely used. Optical interferometry makes useof interference fringes generated by two light beams to accuratelymeasure the three-dimensional shape and structure of the surface of anobject. Thanks to its characteristics of non-contact measurement andhigh measurement accuracy, optical interferometry is widely used tomeasure the fine shape structure of 3D surface of precision industrialproducts such as semiconductor integrated circuits, flat panel displaypanels (LCD, PDP, EL), and MEMS substrates. It is a key supportingtechnology indispensable to the field of precision machining.

With the rapid development of computer data processing and automaticcontrol technologies, great progresses have also been made in theoptical interferometry technology. The NewView8000 developed by ZYGO ofUnited States, the BW-5500 from Nikon of Japan, and the ContourGT-K fromBruker of Germany represent the current top level of three-dimensionaloptical precision measuring instruments. Such a measuring instrument ischaracterized by nanometer-level measurement accuracy, convenientoperation, and fully functional application software. However, becausesuch a measuring instrument adopts a symmetric optical interferencesystem and a stepping scanning method, it also has such shortcomings aslow measurement speed and narrow measurement range. When an opticalinterference system is symmetrical, it indicates that the testingoptical system is symmetrical to the reference optical system. As shownin FIG. 1, which is a schematic diagram of the structure of asymmetrical optical interference system, if it is necessary to expandthe testing range, it is also necessary to expand the reference datum;high precision machining and assembly techniques are required, in orderto design and process a large area of reference datum, which is costly;in addition, there are only a handful of companies around the worldwhich can make large-area reference datums, making it difficult tooperate.

In view of this, it is urgent to address the problems of existingoptical interferometry methods, such as, low measurement speed, narrowmeasurement range, high cost and difficulty in operation.

SUMMARY OF THE INVENTION

The present application is intended to solve technical problems ofexisting optical interferometry methods such as low measurement speed,narrow measurement range, high cost, and difficulty in operation.

To address such problems, the technical solution of the presentapplication provides an asymmetric optical interferometry method, whichcomprises the following steps that: An incident light source is splitinto two beams via a spectroscope, and the two beams are respectivelyprojected onto the surface of an object to be tested and the surface ofa reference mirror; these two beams then respectively pass through afirst imaging lens having a larger area on the side of the object to betested, and a second imaging lens having a smaller area on the side ofthe reference mirror, and overlap on a photoelectric sensor afterpassing through a third imaging lens to form at least one interferenceimage; the magnification of the first imaging lens is smaller than thatof the second imaging lens; The corresponding interference image isinput into a computer to obtain a signal of the correspondinginterference image;

The signal of corresponding interference image is parsed to obtain athree-dimensional shape of the surface of the object to be tested.

In the above-mentioned technical solution, a plurality of interferenceimages can be formed on the photoelectric sensor by simultaneouslyadjusting the distance between the second imaging lens and thespectroscope, and the distance between the reference mirror and thespectroscope, and maintaining the distance between the second imaginglens and the reference mirror unchanged, so as to alter the optical pathdifference between the optical path on the side of the reference mirrorand the one on the side of the object to be tested. In theabove-mentioned technical solution, a collimating lens, a right anglesteering mirror and a 180-degree retroreflecting mirror are insertedbetween the second imaging lens and the reference mirror; the distancebetween the right angle steering mirror and the 180-degreeretroreflecting mirror is adjusted, to compensate for the optical pathdifference between the optical path on the side of the reference mirrorand the one on the side of the object to be tested due to changes madeto the imaging position of the object to be tested. In theabove-mentioned technical solution, the photodetector is an area-arraycamera photoelectric sensor.

In the above-mentioned technical solution, the signal of thecorresponding interference image is parsed by a phase-shifting algorithmor a white light interferometry method.

The present application also provides an asymmetric opticalinterferometry device, which comprises a spectroscope, an object to betested, a first imaging lens having a larger area on the side of theobject to be tested, a reference mirror, a second imaging lens having asmaller area on the side of the reference mirror, a third imaging lens,and a photoelectric sensor, wherein the magnification of the firstimaging lens is smaller than that of the second imaging lens;

An incident light source is split into two beams via a spectroscope, andthe two beams are respectively projected onto the surface of the objectto be tested and the surface of the reference mirror; these two beamsthen respectively pass through the first imaging lens on the side of theobject to be tested, and the second imaging lens on the side of thereference mirror, and overlap on the photoelectric sensor after passingthrough a third imaging lens to form at least one interference image.

In the above-mentioned technical solution, the reference mirror islocated at a back focal plane of the second imaging lens.

The present application provides an asymmetric optical interferometrymethod and device, which are simple in operation and have largemeasurement range. According to the asymmetric optical interferometrymethod and device, different magnifications are adopted, and asmall-area reference mirror is used to obtain the interference image ofthe surface of a large-area object to be tested, and then thethree-dimensional shape of the surface of the object to be tested isobtained by parsing one or more interference images. The presentapplication has the advantages of reasonable structure design,convenient operation, low cost and short data sampling time; moreover,the measuring instrument has high anti-interference performance, highmeasurement precision, large measurement scope and high workingstability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structure schematic diagram of a symmetric opticalinterference system;

FIG. 2 is a flow diagram of an asymmetric optical interferometry methodprovided by the present application;

FIG. 3 is a structure schematic diagram of an asymmetric opticalinterferometry device provided by an embodiment 1 of the presentapplication;

FIG. 4 is a structure schematic diagram of an asymmetric opticalinterferometry device provided by an embodiment 2 of the presentapplication;

FIG. 5 is an image of a reference mirror M1 formed by using a measuringinstrument which is located at the side of the reference mirror M1 aloneand provided by the present application;

FIG. 6 is an image of an object to be tested 2 formed by using ameasuring instrument which is located at the side of the object to betested 2 alone and provided by the present application;

FIG. 7 is an interference image formed by simultaneously using measuringinstruments which are located at the side of the reference mirror M1 andthe side of the object to be tested 2 and provided by the presentapplication.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present application provides an asymmetric optical interferometrymethod and device, which are simple in measuring operation and largemeasurement range with a diameter up to 1000 mm. According to theasymmetric optical interferometry method and device, differentmagnifications are adopted, and a small-area reference mirror is used toobtain an interference image of the surface of a large-area object to betested, and then the three-dimensional shape of the surface of theobject to be tested is obtained by parsing one or more interferenceimages. The present application has the advantages of reasonablestructure design, convenient operation, low cost and short data samplingtime; moreover, the measuring instrument has high anti-interferenceperformance, high measurement precision, large measurement scope andhigh working stability.

The present application is described in detail below in conjunction withthe drawings and specific implementation mode.

The invention provides an asymmetric optical interferometry method, asshown in FIG. 2, comprising the following steps that:

S1. an incident light source is split into two beams, namely a testinglight and a reference light, via a spectroscope (beam splitter), and thetwo beams are respectively projected onto the surface of an object to betested and the surface of a reference mirror, wherein the testing lightreflected by the surface of the object to be tested passes through afirst imaging lens having a larger area on the side of the object to betested, and the reference light reflected by the surface of thereference mirror passes through a second imaging lens having a smallerarea on the side of the reference mirror; and finally the testing lightand the reference light overlap on an area-array camera photoelectricsensor after passing through a third imaging lens to form at least oneinterference image.

The magnification of the first imaging lens on the side of the object tobe tested is smaller than that of the second imaging lens on the side ofthe reference mirror.

S2. the corresponding interference image is input into a computer toobtain a signal of the corresponding interference image;

S3. the signal of the corresponding interference image is parsed byusing a phase shift algorithm or a white light interferometry method toobtain a three-dimensional shape of the surface of the object to betested.

A plurality of interference images can be formed on the photoelectricsensor, by simultaneously adjusting the distance between the secondimaging lens and the spectroscope, and the distance between thereference mirror and the spectroscope, and maintaining the distancebetween the second imaging lens and the reference mirror unchanged, soas to alter the optical path difference between the optical path on theside of the reference mirror and the optical path on the side of theobject to be tested.

A collimating lens, a right angle steering mirror and a 180-degreeretroreflecting mirror are inserted between the second imaging lens andthe reference mirror; the distance between the right angle steeringmirror and the 180-degree retroreflecting mirror is adjusted, tocompensate for the optical path difference between the optical path onthe side of the reference mirror and the optical path on the side of theobject to be tested due to changes made to the imaging position of theobject to be tested.

Embodiment 1

Embodiment 1 of the present application provides an asymmetric opticalinterferometry device, as shown in FIG. 3, including a spectroscope 1,an object to be tested 2, a first imaging lens L1 having a large area onthe side of the object to be tested 2, and a reference mirror M1(located at the back focal plane of the second imaging lens L2), asecond imaging lens L2 having a smaller area on the side of thereference mirror M1, a third imaging lens L3, and an area-array cameraphotoelectric sensor 3. The magnification of the first imaging lens L1is smaller than that of the second imaging lens L2; an incident lightsource (a light source with a high coherence such as laser light) issplit into two beams, namely the testing light and the reference light,by the spectroscope 1, and the two beams are respectively projected ontothe surface of the object to be tested 2 and the surface of thereference mirror M1, wherein the testing light reflected by the surfaceof the object to be tested 2 passes through the first imaging lens L1,and the reference light reflected by the surface of the reference mirrorM1 passes through the second imaging lens L2; and finally the testinglight and the reference light overlap on the area-array cameraphotoelectric sensor after passing through the third imaging lens L3 toform at least one interference image.

The first imaging lens L1 on the side of the object to be tested 2 has asmall magnification, allowing the surface of the large-area object to betested to be imaged onto the surface of the area-array cameraphotoelectric sensor 3, and conversely, the second imaging lens on theside of the reference mirror M1 L2 has a large magnification, and thesurface of the small-area reference mirror M1 can be imaged onto thesurface of the area-array camera photoelectric sensor 3, and because animage of the surface of the object to be tested 2 and an image of thesurface of the reference mirror M1 on the surface of the area-arraycamera photoelectric sensor 3 are completely overlapped, an interferenceimage can be formed on the surface of the area-array cameraphotoelectric sensor 3, to realize large-area interference imaging of ahigh-coherence light source.

Embodiment 2

By further optimizing the embodiment 1, embodiment 2 provides anasymmetric optical interferometry device, which may use a light sourcewith high coherence such as laser as an incident light source, or alight source with lower coherence such as halogen white light. As shownin FIG. 4, it includes a spectroscope 1, an object to be tested 2, afirst imaging lens L1 having a larger area on the side of the object tobe tested 2, a second imaging lens L2 having a smaller area on the sideof the reference mirror M1, a third imaging lens L3, the collimator lensL4 having a small area on the side of the reference mirror M1, anarea-array camera photoelectric sensor 3, a right angle steering mirrorM2, and an 180-degree retroreflecting mirror M3.

The right angle steering mirror M2 and the 180-degree retroreflectingmirror M3 are configured between the collimating lens L4 and thereference mirror M1, the distance between the right angle steeringmirror M2 and the 180-degree retroreflecting mirror M3 is adjusted tocompensate for the optical path difference between the optical path onthe side of the reference mirror M1 and the optical path on the side ofthe object to be tested 2 due to changes made to the imaging position ofthe object to be tested 2. The imaging position of the object to betested 2 is closely related to the measurement range. If the distancebetween the object to be tested 2 and the first imaging lens L1 isincreased, the measurement range of the object to be tested 2 as well asand the optical path on the side of the object to be tested 2 will alsobe increased. Therefore, it is necessary to ensure that the optical pathon the side of the object to be tested 2 and the optical path on theside of the reference mirror M1 are the same, in order to achievelarge-area interference imaging of a low coherence light source.

The implementation of the asymmetric optical interferometry methodprovided by the present application is described below by using asemiconductor laser source as an incident light source. The centerwavelength of the used semiconductor laser source is 670 nm, and theoutput power is 0.8 MW. The reference mirror M1 has a diameter of 3.15mm and surface precision of λ/20 (@633 nm); the magnification of theside of the reference mirror M1 is 1.0; the object to be tested 2 isflat glass, with a diameter of 45.0 mm; the magnification of the side ofthe object to be tested is 0.07, The used area-array sensorphotoelectric sensor is an area-array CCD industrial camera produced byOpteon Corporation of the United States.

FIG. 5 illustrates the image of the reference mirror M1 formed by usingthe measuring instrument on the side of the reference mirror M1 alone;FIG. 6 shows the image of the object to be tested 2 formed by using themeasuring instrument on the side of the object to be tested 2 alone.FIG. 7 illustrates the interference image formed by using the measuringinstruments on both the side of the reference mirror M1 and the side ofthe object to be tested 2.

It can be seen from FIG. 5 and FIG. 6 that by using two differentmagnifications, the reference mirror and the object to be tested formimages of the same size on the surface of the area-array CCD industrialcamera; as can be seen from FIG. 7, if the measuring instruments on boththe side of the reference mirror M1 and the side of the object 2 areused simultaneously, the two images will overlap on the surface of thearea-array CCD industrial camera to form an interference image.

The present application is not limited to the above-mentioned preferredembodiments, and any structural variations made by anyone inspired bythe present application and forming technical solutions the same as orsimilar to those of the present application shall fall within the scopeof protection of the present application.

1. An asymmetric optical interferometry method, which is characterizedby comprising the following steps that: an incident light source issplit into two beams via a spectroscope; the two beams are respectivelyprojected onto the surface of an object to be tested and the surface ofa reference mirror; and the two beams respectively pass through a firstimaging lens having a larger area on the side of the object to betested, and a second imaging lens having a smaller area on the side ofthe reference mirror, and then overlap on the photoelectric sensor afterpassing through a third imaging lens to form at least one interferenceimage, and the magnification of the first imaging lens is smaller thanthat of the second; the corresponding interference image is input into acomputer to obtain a signal of the corresponding interference image; thesignal of the corresponding interference image is parsed to obtain athree-dimensional shape of the surface of the object to be tested.
 2. Anasymmetric optical interferometry method of claim 1, which ischaracterized in that a plurality of interference images can be formedon a photoelectric sensor, by simultaneously adjusting the distancebetween the second imaging lens and the spectroscope, and the distancebetween the reference mirror and the spectroscope, and maintaining thedistance between the second imaging lens and the reference mirrorunchanged, so as to alter the optical path difference between theoptical path on the side of the reference mirror and the optical path onthe side of the object to be tested.
 3. An asymmetric opticalinterferometry method of claim 2, which is characterized in that acollimating lens, a right angle steering mirror and a 180-degreeretroreflecting mirror are inserted between the second imaging lens andthe reference mirror; the distance between the right angle steeringmirror and the 180-degree retroreflecting mirror is adjusted tocompensate for the optical path difference between the optical path onthe side of the reference mirror and the optical path on the side of theobject to be tested due to changes made to the imaging position of theobject to be tested.
 4. An asymmetric optical interferometry method ofclaim 1, which is characterized in that the photodetector is anarea-array camera photoelectric sensor.
 5. An asymmetric opticalinterferometry method of claim 1, which is characterized in that signalof the corresponding interference image is parsed by a phase shiftalgorithm or a white light interferometry.
 6. An asymmetric opticalinterferometric device, which is characterized by comprising aspectroscope, an object to be tested, a first imaging lens having alarger area on the side of the object to be tested, a reference mirror,a second imaging lens having a smaller area on the side of the referencemirror, a third imaging lens, and a photoelectric sensor, wherein themagnification of the first imaging lens is smaller than that of thesecond imaging lens; an incident light source is split into two beamsvia a spectroscope, which are respectively projected onto the surface ofthe object to be tested and the surface of the reference mirror; thesetwo beams then respectively pass through the first imaging lens on theside of the object to be tested, and the second imaging lens on the sideof the reference mirror, and overlap on the photoelectric sensor afterpassing through a third imaging lens to form at least one interferenceimage.
 7. An asymmetric optical interferometry device of claim 6, whichis characterized in that the reference mirror is positioned at a backfocal plane of the second imaging lens.